In the manufacture of various grades of paper, it is common to incorporate fillers and opacifiers into the paper in order to improve the optical properties of the paper. Increasing the opacity of the paper is particularly desirable for writing papers and tissue. In this regard, it is necessary to adequately retain the filler materials on the fibers in order to ensure that the filler materials remain with the fibers during the pulping and forming stages of the papermaking process. Commercially this is usually achieved by flocculating the filler with a cationic starch or similar coagulant and adding a cationic or anionic retention aid. Even with the use of such process aids, however, filler losses can be significant, especially in low basis weight products or high speed processes. Poor retention of fillers can cause plugging of papermaking felts, fouling of the white water system, and increased raw materials costs.
To this end, calcium carbonate and other suitable salts have been incorporated into the fiber walls and works reasonably well. However, although there are other metal compounds that provide significantly improved hiding power, suitable methods for incorporating such compounds into the fiber walls have not been developed. For example, titanium dioxide, when compared to calcium carbonate, has a higher refractive index and greater scattering power and is about ten times more efficient on a per pound basis. However, no suitable method has yet been devised to incorporate these other compounds into the cell walls of papermaking fibers.
Therefore a need exists to produce a pulp where a high refractive index pigment, such as titanium dioxide, can be incorporated into the fiber cell walls to produce fibers having a very high refractive index and having a very high retention of filler in high shear processes or products of low basis weight as well as good retention of physical properties.